1. Field of the Invention
The present invention relates to a vacuum platen for media sheets, having a sheet support wall formed with a plurality of perforations, and a number of chambers formed on a bottom side of the sheet support wall, wherein each of said chambers is directly connected to a vacuum source.
2. Description of Background Art
An example of a vacuum platen of this type is described in EP 1 182 040 B1.
Such vacuum platens are used, for example, in printers or copiers for holding media sheets in a flat condition on the surface of the platen. Since a vacuum is created in each of the chambers by a vacuum source or sources, ambient air will be drawn-in through the perforations of the sheet support wall, so that a sheet that has been placed onto the platen will be attracted against the sheet support wall. In general, it is desired that the platen is capable of holding media sheets of different formats. Thus, when a small format sheet is disposed on the platen, not all of the perforations of the sheet support wall will be covered by the sheet, but a relatively large number of perforations will be left open, e.g. at the lateral sides of the platen. Since a relatively large amount of air will be drawn-in through these open perforations, the vacuum underneath the sheet support plate is likely to break down when the power of the vacuum source is not sufficient. This effect is mitigated by dividing the space below the sheet support wall into the plurality of chambers that are individually connected to the vacuum source, so that it is easier to maintain the vacuum in those chambers for which most of the perforations are covered by the sheet.
Another measure to limit the necessary power of the vacuum source and, accordingly, to limit the energy consumption, is to reduce the size of the perforations, so that less air will be drawn in even when the perforations are open. However, with decreasing size of the perforations, there is an increased risk that the air flow through the perforations causes a disagreeable whistling noise.
It has been attempted to avoid this noise by appropriately selecting the shapes of the perforations and/or by carefully machining the edges of the perforations. These measures, however, increase the production costs and conflict with the objective to reduce the size of the perforations.